1. Field of the Invention
This invention relates to a torque converter, and particularly to a lock-up clutch for use in a lock-up torque converter.
2. Related Background Art
FIG. 1 and 2 of the accompanying drawings illustrate the function of the clutch facing 3 of a lock-up clutch 2 in a torque converter 1.
The lock-up clutch 2 urges the facing 3 against a converter cover 4 by pressure rise in the torque converter 1 and transmits power by a frictional force. The facing 3, as shown in FIG. 2, is of a circular ring shape in which inner and outer marginal edges a and b form smooth concentric circles.
Such a clutch facing 3 assumes a very high temperature due to friction heat created when it is slide-coupled to the converter cover 4 and therefore, it is necessary to cool the slide surface thereof effectively. Applicant, paying attention to this problem, has proposed a facing structure improved in cooling characteristic in Japanese Patent Application No. 63-254772 (laid-open under Publication No. 2-102963). FIG. 12 of the accompanying drawings shows a facing 3 thus proposed as it is mounted on a core plate 5. In FIG. 12, the reference numeral 20 designates a plurality of oil grooves radially formed in the back of the facing 3 (the surface thereof adhesively secured to the core plate 5). Oil circulating in these oil grooves is absorbed by the facing 3 and functions as a coolant during the frictional engagement of the facing with the converter cover.
In the above-mentioned patent application, oil grooves are formed in the facing itself, but it has been found that since the grooves are formed by severing fiber which is the material of the facing, this facing structure has the disadvantage that the shearing strength of the facing is reduced. Therefore, it has become clear that when an excessively great torque is applied to the lock-up clutch, the peeling or destruction of the facing may occur from the oil groove portions.
Recently, there has been a trend toward increased input torque resulting from the higher output of an engine, with the result that the amount of heat to which the clutch facing is subjected increases. To cope with this, it is effective to increase the num and cross-sectional areas of the oil grooves, but too great a quantity of oil flowing in the grooves may result in a decrease in the pressure difference between the front and rear of a clutch plate and thus, a reduction in the capacity of the clutch.